Energy may be harvested from the movement of body joints of humans and other animals by converting mechanical energy derived from such movement to electrical energy. Activities where body joints move repeatedly, such as walking, jogging, and running, for example, present opportunities to continually harvest energy from moving body joints. In some energy harvesting devices and methods, a generator driven by joint motion is coupled to an electrical load. Since the instantaneous mechanical power provided by body joints during repetitive or cyclical activities typically varies over the period of each cycle, both the electrical power supplied to the load and the forces applied to the body joint may be time-varying over each cycle. In some circumstances, the variations of delivered electrical power and/or forces applied to body joints that occur in this arrangement may be undesirable, for reasons such as efficiency and user comfort.
A knee joint 10 is shown in FIG. 1, in which a femur 12 and tibia 14 are depicted in an extended position. Also shown is a position of the knee joint where the femur has flexed to the right, to position 12A, as if the person were sitting down or squatting. The medial collateral ligament insertion 16 on the femur 12 is shown to move along arc 17 during flexion of the knee joint 10, the center of this ligament insertion passing through corresponding numbered points 1-6. Also, during flexion, the medial condyle 18 rolls and slides such that its geometrical center shifts, shown by corresponding numbered points 1-6 indicated by arrow 20. During extension and flexion the knee joint does not rotate on a fixed axis, but moves through both a rolling and sliding motion along the interface between the lower surface 22 of the femoral condyles and tibial plateau 24.
This means that a simple single pivot hinge does not provide a good approximation of knee kinematics. In the context of brace design, a single pivot system would result in migration of the brace as a result of the incongruent motion of the knee and the brace.
Tracking a point 20 at the approximate spherical center of the femoral condyles during extension and flexion results in an approximately straight line trajectory along the tibial plateau. This trajectory is referred to as femoral rollback. Being an approximate straight line trajectory, this has led knee brace and prosthetics designers to mimic this by implementing a Chebyshev four-bar linkage 29, shown schematically in FIG. 2. The linkage 29 has a lower link 30, two identical cross links 32, 34 and an upper link 36, all connected at their ends with pivot points 38. As the upper link 36 turns either to the left or the right, the constraining action of the cross links 32, 34 result in the center point 40 of the upper link moving in an approximately straight line 42. In order to achieve the approximate straight line trajectory, the three-way length ratio of the lower link 30 to cross link 32, 34 to upper link 36 must be 2:2.5:1. Conventionally, the lower link 30 is oriented horizontally.
FIG. 3 shows a knee joint with femur 12 and tibia 14, in which the femur would rotate to the right during flexion. A four-bar linkage 50 is superimposed on the joint. The side link 52 approximately aligns with the PCL (posterior cruciate ligament) 54, and can be referred to as the PCL link. The side link 56 approximately aligns with the ACL (anterior cruciate ligament) 58 and can be referred to as the ACL link. The crossing side links 52, 56 loosely resemble the ACL and PCL ligaments in both geometry and kinematic function. The upper link 57 connects to the upper ends of the ACL link 56 and the PCL link 52, and as it is in a fixed orientation relative to the femur, it may be called the femur link. The position of the lower link 59 is fixed relative to the tibia and angled downwards posteriorly, approximately parallel with the typical 10-15° incline of the tibial plateau. The lower link 59 may be called the tibia link.
Knee prosthetics use four-bar linkage mechanisms between a shin brace and a thigh brace, but these linkages have a gearing ratio that declines steeply as the knee is flexed. The angular velocity is low at deep flexion, and there is a high angular velocity at leg extension. This would lead to a sharp periodic peak in noise in an energy-harvesting gearbox.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.